build: keep sccache prefix when CUDA_PYTHON_TOOLCHAIN selects a compiler

cuda_bindings/build_hooks.py

@@ -127,18 +127,28 @@ def _resolve_toolchain_name():
     return name, allowed, cc, cxx, explicit
 
 
+def _with_sccache(current, compiler):
+    """Keep CC="sccache cc" as CC="sccache clang" when the toolchain picks a compiler."""
+    if current:
+        launcher = current.split()[0]
+        if os.path.basename(launcher) == "sccache":
+            return f"{launcher} {compiler}"
+    return compiler
+
+
 def _apply_toolchain_env(cc, cxx, explicit):
     """Set CC/CXX/LDSHARED for an explicitly-chosen toolchain.
 
     The default path (CUDA_PYTHON_TOOLCHAIN unset) intentionally
     does not touch the env, so an externally-set compiler (e.g.
     CC="sccache cc" in CI) keeps working. An explicit CUDA_PYTHON_TOOLCHAIN
-    override (incl. =gnu) governs the compiler and overrides CC/CXX/LDSHARED.
+    override (incl. =gnu) governs the compiler. An existing sccache prefix
+    is kept (CC="sccache cc" + llvm -> CC="sccache clang").
     """
     if explicit and cc is not None:
-        os.environ["CC"] = cc
-        os.environ["CXX"] = cxx
-        os.environ["LDSHARED"] = f"{cxx} -shared"
+        os.environ["CC"] = _with_sccache(os.environ.get("CC", ""), cc)
+        os.environ["CXX"] = _with_sccache(os.environ.get("CXX", ""), cxx)
+        os.environ["LDSHARED"] = f"{os.environ['CXX']} -shared"
 
 
 def _check_toolchain_available(name):

cuda_bindings/tests/test_build_hooks.py

@@ -135,6 +135,18 @@ def test_gnu_sets_env_and_flags(self, monkeypatch):
         assert "-fpermissive" in cargs
         assert "-fno-var-tracking-assignments" in cargs
 
+    @pytest.mark.agent_authored(model="grok-4.6")
+    def test_llvm_keeps_sccache_prefix(self, monkeypatch):
+        if sys.platform == "win32":
+            pytest.skip("llvm only valid on Linux")
+        monkeypatch.setenv("CUDA_PYTHON_TOOLCHAIN", "llvm")
+        monkeypatch.setenv("CC", "sccache cc")
+        monkeypatch.setenv("CXX", "sccache c++")
+        _name, _cc, _cxx, _cargs, _largs = build_hooks._resolve_toolchain()
+        assert os.environ["CC"] == "sccache clang"
+        assert os.environ["CXX"] == "sccache clang++"
+        assert os.environ["LDSHARED"] == "sccache clang++ -shared"
+
     @pytest.mark.agent_authored(model="glm-5.2")
     def test_gnu_keeps_gcc_only_flags(self, monkeypatch):
         if sys.platform == "win32":

cuda_core/build_hooks.py

@@ -126,18 +126,28 @@ def _resolve_toolchain_name():
     return name, allowed, cc, cxx, explicit
 
 
+def _with_sccache(current, compiler):
+    """Keep CC="sccache cc" as CC="sccache clang" when the toolchain picks a compiler."""
+    if current:
+        launcher = current.split()[0]
+        if os.path.basename(launcher) == "sccache":
+            return f"{launcher} {compiler}"
+    return compiler
+
+
 def _apply_toolchain_env(cc, cxx, explicit):
     """Set CC/CXX/LDSHARED for an explicitly-chosen toolchain.
 
     The default path (CUDA_PYTHON_TOOLCHAIN unset) intentionally
     does not touch the env, so an externally-set compiler (e.g.
     CC="sccache cc" in CI) keeps working. An explicit CUDA_PYTHON_TOOLCHAIN
-    override (incl. =gnu) governs the compiler and overrides CC/CXX/LDSHARED.
+    override (incl. =gnu) governs the compiler. An existing sccache prefix
+    is kept (CC="sccache cc" + llvm -> CC="sccache clang").
     """
     if explicit and cc is not None:
-        os.environ["CC"] = cc
-        os.environ["CXX"] = cxx
-        os.environ["LDSHARED"] = f"{cxx} -shared"
+        os.environ["CC"] = _with_sccache(os.environ.get("CC", ""), cc)
+        os.environ["CXX"] = _with_sccache(os.environ.get("CXX", ""), cxx)
+        os.environ["LDSHARED"] = f"{os.environ['CXX']} -shared"
 
 
 def _check_toolchain_available(name):

cuda_core/tests/test_build_hooks.py

@@ -696,16 +696,17 @@ def test_gnu_sets_env_and_flags(self, monkeypatch):
         assert "-fpermissive" not in cargs  # cuda.core gnu flags don't include it; bindings do
         assert "-fno-var-tracking-assignments" not in cargs
 
-    @pytest.mark.agent_authored(model="glm-5.2")
-    def test_llvm_overrides_external_cc(self, monkeypatch):
+    @pytest.mark.agent_authored(model="grok-4.6")
+    def test_llvm_keeps_sccache_prefix(self, monkeypatch):
         if sys.platform == "win32":
             pytest.skip("llvm only valid on Linux")
-        # An explicit non-default toolchain governs the compiler, so a stale
-        # external CC (e.g. "sccache cc") is replaced, not kept.
         monkeypatch.setenv("CUDA_PYTHON_TOOLCHAIN", "llvm")
         monkeypatch.setenv("CC", "sccache cc")
+        monkeypatch.setenv("CXX", "sccache c++")
         _name, _cc, _cxx, _cargs, _largs = build_hooks._resolve_toolchain()
-        assert os.environ["CC"] == "clang"
+        assert os.environ["CC"] == "sccache clang"
+        assert os.environ["CXX"] == "sccache clang++"
+        assert os.environ["LDSHARED"] == "sccache clang++ -shared"
 
 
 class TestCheckToolchainAvailable: